This invention relates generally to the control of electrochemical manufacturing processes. More particularly, the present invention relates to a process control system for the operation of a sodium hydrosulfite electrolytic cell to produce concentrated hydrosulfite solutions at high current densities.
The controlled feeding of chemicals in solutions from holding tanks into reactors, process streams or treatment streams is old and well known. The simpliest techniques employ a chemical feed pump with an on/off switch and transfer lines through which move the chemical or solution. This technique requires the presence of a human operator to monitor the transfer. Electromechanical controllers have also been used to determine when chemicals for solutions should be transferred. This approach either measures a property or properties in an end-use stream or automatically times when a desired chemical should be fed. Sensors detect either the proper time or the measured property and generate a signal to initiate pumping action. The pumping action is ceased when the measured property has changed to a desired level or a predetermined time has expired. This approach reduces the need for constant human monitoring, but does not necessarily monitor the actual operation of the feed pump should a feed pump fail for any of a variety of reasons. Later generation controllers have monitored a multiplicity of chemical processes. Controllers add a multiplicity of adjustors or actuators to adjust the control process parameters simultaneously with the monitoring. Alarms are provided whenever the adjustors fail to be self correcting.
A more specialized problem occurs when attempting to provide alkali metal hydrosulfite solutions to industrial customers via on-site production, as opposed to a large centralized production facility that employs shipment of the product to remote using sites. On-site production of the desired product decreases because of the loss of economy of scale obtained when producing the product in large electrochemical facilities. This advantage is also decreased because of the increased manual labor required to operate several on-site production facilities.
The commercial electrolytic production of alkali metal hydrosulfite solutions presents even more system specific challenges. The product must be produced with a consistent composition at varying rates of production to match demand. To accomplish this, the physical and chemical properties of process streams within the electrolytic cell must be carefully controlled. In the event of equipment failure at the remote operating sites, the required sequence of shutdown steps must be taken quickly to avoid damage to the process equipment and the environment, as well as to protect the human operators. The production system must have some method to permit remote access to the system control if the production facilities will be operated at a number of remote locations on the sites where the product alkali metal hydroxide solutions are used.
These inherent disadvantages and problems can be overcome by the use of a process control system which achieves total process control with a single microcomputer that is used to perform multiple functions which permit automatic decision making and process adjustment or shutdown to occur without the need for the presence of a local operator.